It is previously known in this Art to permanently anchor dental prostheses in the jawbone tissue. The method which has proved to result in the greatest anchorage stability--and the only method which has proved to afford a truly permanent anchorage in the bone tissue--is the so-called osseointegration method, developed by Professor Per-Ingvar Br.ang.nemark et. al. in Gothenburg, Sweden. The unique feature of this method is that the anchorage unit (the fixture) may be implanted, both very accurately and atraumatically, directly in the bone tissue without the necessity of interposed bonding tissue. This direct contact between fixture and bone tissue makes for optimum conditions for lasting heating by osseointegration.
The anchorage unit (the fixture) is in the form of a screw, preferably of titanium, and is implanted such that the upper portion of the screw is located flush with or slighly beneath the surface of the jawbone. This first operation is followed by a dentally unloaded healing period of a critical length, during which the screw is covered by intact mucosa. During this healing phase, the bone tissue grows onto and forms a unit with the implanted fixture. In the subsequent operation, the fixture is exposed and a spacer, also preferably made of titanium, is disposed on the fixture by means of a spacer screw. The dental prosthesis in the form of a crown or bridge construction, is then anchored in place by means of a locking screw which positionally fixes the spacer screw. This method has been successfully employed clinically for 20 years in conjunction with jawbone-anchored bridge constructions, and development is now underway concerning the anchorage of prostheses to other parts of the skeleton (extra-oral prostheses).
The unique properties of this method thus derive from the fact that the fixture is wholly incorporated in the bone tissue. This implies that the dental prosthesis will be relatively rigidly anchored in the jawbone, as opposed to, for example, the nature tooth which is more elastically supported. Such a rigid anchorage entails that the oral functional stresses are transmitted without attenuation to the dental prosthesis (the crown or bridge construction) and its anchorage (the spacer and the fixture), which may give rise to excessively high stresses in both the bone tissue anchorage and in the different components of the dental prosthesis. Even if the osseointegration method possesses uniquely advantageous properties as regards the risk of disengagement arising out of overloading, it is, of course nevertheless desirable to reduce, as far as is possible, the levels of force stresses. As a result, fracture of the prosthesis components can be avoided, at the same time as these may be dimensioned in such a manner as requires as little space as possible.
Concerning the oral functional and mastication forces, the velocity of forces and the mass involved are so insignificant that the damping effect of the elastic suspension on transmission of energy per unit of time should be of but marginal importance. Of considerably greater importance is the deflection which may be realized by elastic suspension of one or more, (individual or interconnected), jawbone-anchored crowns in residual bite with natural teeth. Depending upon the degree of elasticity and, thereby, deflection, the occlusal forces can be distributed between the jawbone-anchored units and the natural teeth in a controllable manner. A harmonically adapted degree of elasticity might well also contribute to an optimum level of oral comfort for the patient.
Not least in those cases where one or more jawbone-anchored units are connected to the natural teeth of the residual bite, a pattern of deflection of the jawbone-anchored unit or units which nicely approximates that of the natural teeth should be striven for. Probably, this will result in a considerably more favorable stress distribution throughout both the jawbone-anchored fixtures and across the superstructure/bridge construction which is connected between the fixture and the natural teeth.
Even in those cases where a bridge construction is anchored in the jawbone of a completely edentulous jaw, there may be a need to increase the patient's oral comfort by means of a gentler, less inflexible occlusion realized by means of an elastic element.
Also in extra-oral prostheses, for example joint prostheses, there is a need in this Art for a more elastic suspension of the prostheses in order to increase the feeling of comfort and, above all, to provide for a greater degree of inherent margin to destructive energy transmission level in that the elastic suspension, by deflection, on the one hand automatically leads to less force absorption over the region of the prosthesis and, instead, greater force is absorbed by, for example, the natural extremities, and, on the other hand considerably retards the velocity of the applied force. The possibility of elastic deflection also increases the time margin for the onset of the reflex protective reactions of the organism.